About a third of the world's population is infected with Mycobacterium tuberculosis (MTB). In a stand off that may last decades during the period of clinical latency, a population of MTB persists in a state of apparent bacteriostasis until the host's ability to restrict growth of the pathogen is reduced by declining cell-mediated immunity. Then bacterial replication can resume and reactivation of latent foci leads to clinical disease in about 10% of the immune-competent individuals infected with MTB. Drug therapy of active TB takes 6 to 9 months. Premature termination of therapy decreases its success rate and leads to the development and spread of drug resistant and multi-drug resistant MTB. Drugs for treating active TB are relatively ineffective against MTB in the latent phase of the infection and against non-replicating MTB. New drugs that are active against non-replicating MTB might shorten drug therapy of active TB and also allow the treatment of latently infected individuals that are at high risk to develop active TB. Respiration is fundamental for growth of most bacterial species and also for survival during bacteriostasis. Respiratory chains that occur in MTB but not in humans might be suitable targets for the development of novel anti-mycobacterial drugs that are active against persisting as well as growing bacteria. The goal of this project is to determine the importance of these (alternative) respiratory chains for MTB pathogenesis. We will determine how the energy metabolism of MTB adapts to environments encountered within the host with experiments that monitor the expression levels of genes encoding respiratory enzymes. Using transposon mutants we will test whether alternative respiratory chains are important for pathogenesis of MTB in mice. We will also investigate whether respiratory chains with a low bioenergetic efficiency are important for the pathogen's ability to metabolize highly reduced carbon sources like fatty acids and whether anaerobic respiration is essential for MTB to survive hypoxia.